QOS Best Practices?

[GLHEC]

Is there a best practice for mapping dscp or cos to voip control and voip media traffic? I am looking for a doc that says voice control in most cases should receive dscp X and cos of X. The same for voip control. I understand that it could vary on each network but there must be a general idea? I am not looking for commands and it doesnt neccessarily need to be cisco, I am just looking for a "best practices".
thanks

 

[ADB100]

There are various Cisco AVVID best practise documents on CCO but generally the following is 'normal' for Cisco VoIP networks:

Voice Signalling : CoS 3, DSCP 26 (AF31)
Voice Media : CoS 5, DSCP 46 (EF)

On the ingress when trusting CoS (or IP Precedence) the granularity of DSCP is not available, but due to the way QoS is implemented in most Cisco switches every packet MUST have an internal DSCP value. When a port is trusted for CoS (or IP Precedence) there must be a mapping between the CoS and the DSCP. The Cisco defaults are normally changed to fit in line with IEFT recommendations to the following:

3550G-Access(config)#mls qos map cos-dscp 0 8 16 26 34 46 48 56
To verify, enter the following command (shown with its associated output):

3550G-Access#show mls qos maps
Cos-dscp map:
cos: 0 1 2 3 4 5 6 7
----------------------------
dscp: 0 8 16 26 34 46 48 56


When a packet is scheduled at the egress port it is placed into a queue based on its CoS value; this is the egress DSCP to CoS mapping. Here the internal DSCP values are mapped to a CoS value - this is generally a range of DSCP values mapping to a single CoS value depending on the hardware platform. i.e. DSCP 0-7 map to CoS 0, DSCP 8-15 map to CoS 1, DSCP 16-23 map to CoS 2 etc. On a Catalyst 2950 the following DSCP to CoS mappings can exist:

cat-2950#sho mls qos maps

Dscp-cos map:
dscp: 0 8 10 16 18 24 26 32 34 40 46 48 56
-----------------------------------------------
cos: 0 1 1 2 2 3 3 4 4 5 5 6 7

On a Catalyst 4000 with a Sup3 the following is true:

4006-SUPIII-Access#show qos map dscp tx-queue
DSCP-TxQueue Mapping Table (dscp = d1d2)
d1 : d2 0 1 2 3 4 5 6 7 8 9
-------------------------------------
0 : 01 01 01 01 01 01 01 01 01 01
1 : 01 01 01 01 01 01 02 02 02 02
2 : 02 02 02 02 02 02 02 02 02 02
3 : 02 02 03 03 03 03 03 03 03 03
4 : 03 03 03 03 03 03 03 03 04 04
5 : 04 04 04 04 04 04 04 04 04 04
6 : 04 04 04 04


You have to look at each platform individually with regards to QoS since they all vary considerably; the Catalyst 6500 being the worse since each module can have different queueing techniques....

Have a search on CCO for AVVID best practises.

Andy

 

[GLHEC]

Thanks for the info. I see control is COS3 and media is COS5. I was working on a 3550 that has 4 queues, with one of them for priority. Having only 1 queue for priority, I think I can only match one COS label to it. This is a problem if I saturate a layer2 trunk upstream (since only media is getting sent as priority). Does anyone have any thoughts on what to do here?
thanks

phone
|
|
3550
|
|- saturated link
|
4500

 

[ADB100]

Generally voice Media traffic for only a very small percentage of traffic, it is just delay sesitive and hence must be prioritised. The egress priority queue on a 3550 is serviced until empty, but only has a relatively small buffer (as long as it is tuned correctly). The idea here is that although the voice media traffic is prioritised it won't actually be taking up a lot of bandwidth.
Only voice media traffic should be placed into a priority queue because of this - i.e. if you placed general data in this queue it would fill the allocated buffers very quickly and data will begin to be dropped.
There is a voice bandwidth calculator on CCO that will work out what bandwidth needs to be reserved for your priority queue for the media traffic, plus what needs to be reserved for control traffic.

Your comment on what CoS can map to a queue is slightly wrong since you can map multiple CoS values to each queue, its just that you don't want to for the priority queue.
Andy

 

[GLHEC]

thanks for the help!